To provide further information on the role of donor immune T cells transferred to CY-treated MCA/76-9 sarcoma-bearing mice and to assess whether the induction of tumor regression was mediated by donor or host T cells, or both, the following experiments were undertaken. Radiation chimeric mice (B6 to 86D2F1 and DBA/2 to B6D2F1 mice) were established. The blood and the lymphoid tissues of these mice expressed the H-2 haplotype of the reconstituting bone marrow; that is, there were no detectable F1 cells remaining in these tissues. About 12 weeks after reconstitution, the mice were injected with sarcoma cells, followed 10 days later by CY and immune F1 hybrid cells. After a further 10 days, the cellular composition of the regressing tumor mass was analyzed using glucose phosphate isomerase (Gpi) as a qualitative marker for the presence of B6 or DBA isozymes (Gpi-1b and 1a, respectively) and monoclonal anti-H-2 antibody for the presence of cells bearing the H-2b or H-2d determinants. It was seen that following electrophoresis of tumor-associated cell extracts that all three bands of the B6, DBA, and the F1 hybrids were expressed. However, on fractionation of the tumor cells into macrophages, neutrophils, and T lymphocytes, it was demonstrated by both the Gpi assay and H-2 typing that macrophages and neutrophils were derived from the bone marrow used to reconstitute the lethally irradiated F1 hybrid mice originally; that is, either from the B6 or DBA marrow, while the T cells were F1 in origin. It is, therefore, likely that donor T-cell immune responses are amplified under these conditions at the tumor site in the absence of host T-cell responses. On day 8 post therapy, the tumor-associated cells (TAC) isolated from F1 or chimeric mice consisted of 60 to 70% T cells and 20 to 30% macrophages. The TAC were cytotoxic in vitro and in a Winn assay. Although the cellular composition and cytotoxic potential of TAC were similar, permanent tumor eradication was seen only in the F1 hybrids. After about 30 days' post therapy, tumors reappeared in almost all chimeric mice. The hypothesis we are currently testing is that suppressor mechanisms are induced in the chimeras and these down-regulate active immunity at the site of regression. (MB)